Court papers reveal that Mr Keays was 27 when he took a post as a junior engineer onboard the 3,140-passenger Caribbean Princess.

It was his first job since graduating from City of Glasgow College.

When he became aware of pollution issues, he secretly used his mobile phone to take photographs and record video footage of key equipment.

'Blatant disregard'

"His concern and fear throughout the video recordings is palpable", the court papers note.

Mr Keays walked off the ship at the next port - Southampton - and immediately reported his findings to the Maritime and Coastguard Agency.

In a letter to US District Judge Patricia A Seitz, Mr Keays told her that his job with Princess Cruise Lines was the "chance of a lifetime".

But he left the ship feeling "hugely disappointed" that there was a "blatant disregard for the protection of the seas and in defiance of the law".

"Thinking back, I had not considered the implications of my response and that my career may be over before it barely started," he said.

"My actions were an automatic response to wrong, when so many others clearly turned a blind eye," he added.

"I genuinely hope that this will be a wake up call for the industry, that my actions will be replicated and empower those with knowledge of these practices to do the right thing, and finally deliberate pollution will become a past shame rather than a continued illegal practice that is unspoken of by many ships crew."

Mr Keays, who is now working in Spain, told The Times that he felt vindicated by the decision of the court.

He said the $1m (£870,000) reward would give him financial security.

'Inexcusable actions'

The Caribbean Princess had been making illegal discharges since 2005, according to the US Department of Justice.

It said the case against Princess Cruise Lines included illegal practices which were found to take place five of its ships.

He said: "Without the courageous act of a junior crewmember to alert authorities to these criminal behaviours of deliberately dumping oil at sea, the global environmental damage caused by the Princess fleet could have been much worse.

"The selflessness of this individual exposed five different ships that embraced a culture of shortcuts and I am pleased at this outcome."

Part of Princess Cruise Lines' $40m (£31.2m) fine will be split between environmental projects in the UK and Florida.

In a statement, the firm said: "We are extremely disappointed about the inexcusable actions of our employees who violated our policies and environmental law when they bypassed our bilge water treatment system and discharged untreated bilge water into the ocean."

It added that management cooperated with the authorities and they launched their own internal investigation.

"Although we had policies and procedures in place, it became apparent they were not fully effective," the statement continued.

"We are very sorry that this happened and have taken additional steps to ensure we meet or exceed all environmental requirements."